The present invention concerns a plastic sheet having a number of recessed and protruded parts prepared by forming recessed parts and protruded parts on a flat sheet. The invention also concerns a two-layered or three-layered double wall sheet prepared by combining the above plastic sheet with another flat plastic sheet or sheets. The invention further concerns a method of producing the above-mentioned plastic sheet having a number of recessed and protruded parts, a method of producing the above-mentioned double wall sheet, and an apparatus for producing them.
There has been known processed article of thermoplastic material called “plastic bubble sheet”, which is produced with a thermoplastic material such as polyethylene and polypropylene by thermal processing (usually, vacuum forming) to form a cap-film having a number of cap-shaped projections and by adhering a flat back film to the base surface of the cap film or adhering a flat liner film to the tops of the caps, or adhering both the back film and the liner film. The product is widely used as the material for cushion-packaging, thermal insulators, or a protective sheet at moving and construction.
There are variations of the plastic bubble sheets in which thicker films, which may be more appropriately called sheets, are used as the constituting material. The products are called “plastic bubble boards” and are also widely used as the building materials, concrete placing panels, protecting boards, or materials for various containers.
The difference between the plastic bubble sheet and the plastic bubble board resides in the difference of thickness of the films or sheets as the components, and the difference of thickness of the product plastic bubble sheet and the plastic bubble board, on which the strength and the rigidity of the products depend, and therefore, the two kinds of products are applied to the appropriate use. In the following description the material films and sheets are represented by a “sheet”, and the product bubble sheets and bubble boards, by a “bubble board”.
The above described bubble board have two grades: the two-layered bubble board prepared by adhering a back sheet to the base of the cap sheet or by adhering a liner sheet to the tops of the caps, and the three-layered bubble board prepared by adhering both the back sheet and the liner sheet. Of the two-layered ones, those having a back sheet have lower rigidity, and therefore, are applied to the use where no bending rigidity is required. Even though the two-layered ones having a liner sheet can be applied to the use where the bending rigidity is required, it is a drawback that the products are not suitable as the protecting sheet, because those who stepped on the sheet may miss their footing. The three-layered ones have the highest bending rigidity and both the surfaces are smooth, and therefore, are suitable for applying to the use where such the features are required and where slipping is out of the question. The weak point of the three-layered bubble boards is, due to the producing process, that warps tend to occur during producing.